Rain Barrel Calculator

Calculate rainwater harvest potential from your roof area. Get gallons per rainstorm and number of barrels needed.

🌧️ Rain Barrel Calculator

Results

Roof Area1,200.00 sq ft
Gallons per 1β€³ Rain748 gal
Collectible per Storm599 gal
Barrels Needed11 barrels

πŸ’‘ How to Calculate Rainwater Collection

One inch of rain on 1 square foot of roof yields 0.623 gallons of water. A typical 30Γ—40 ft roof section produces 748 gallons per inch of rain. Collection efficiency is about 75–85% due to evaporation, splash, and first-flush diversion.

Standard rain barrels hold 55 gallons. Position barrels under downspouts and connect overflow to a garden or drainage area. For larger systems, multiple barrels can be linked together. Collected rainwater is excellent for garden irrigation.

A 30Γ—40 ft roof section with 1 inch of rain at 80% efficiency: 1,200 sq ft Γ— 0.623 Γ— 0.80 = 598 gallons. You would need 11 standard 55-gallon rain barrels to capture it all.

Rain Barrel Calculator FAQ

πŸ”— Related Calculators

Pipe Volume CalculatorCalculate pipe volume by size and material. Get gallons, liters, cubic feet, water weight, and gallons per foot. Supports multiple pipes.Flow Rate CalculatorCalculate flow rate two ways: pipe velocity Γ— area or volume Γ· time. Standard US pipe sizes, GPM, GPH, liters/min, CFS, and daily estimate.CFM CalculatorCalculate CFM airflow by room type. Auto-set ACH, ASHRAE occupant minimum, L/s metric, and duct size recommendation.Square Footage CalculatorCalculate the square footage of any space β€” rectangle, triangle, circle, trapezoid, or rectangular border. Convert to sq m, sq yd, and acres. Estimate project cost with price per sq ft.